Influence of adipose tissue mass on DXA-derived lean soft tissue mass in middle-aged and older women.

The purpose of this study was to examine the influence of fat-free adipose tissue mass (FFAT) on association between dual-energy X-ray absorptiometry (DXA)-derived lean soft tissue mass and skeletal muscle mass (TMM). Forty-one middle-aged and older women were recruited for this study. Percent body fat, total and appendicular fat mass (tFM and aFM, respectively), and total and appendicular lean soft tissue mass (tLM and aLM, respectively) were measured using a DXA. FFAT was calculated based on the methods of a previous study. TMM was estimated from the ultrasound-derived prediction equation. The subjects were separated into three groups based on DXA-determined percent fat: low (n = 12, <25 %), middle (n = 15, ≥25 and <35 %), and high (n = 14, ≥35 %). DXA-derived aLM was greater in high than in middle or low, although ultrasound-estimated TMM was similar among the three groups. There was a strong correlation between aLM and TMM (r = 0.905, p < 0.001). The difference between aLM and TMM was correlated (p < 0.001) with aFM (r = 0.599) and tFM (r = 0.587). After adjusting for FFAT, aLM minus appendicular FFAT was similar among the three groups. aLM minus appendicular FFAT was strongly associated with TMM (r = 0.912, p < 0.001). Our results suggest that DXA-derived aLM accurately predicts TMM when subjects have moderate or lower adipose tissue mass. However, FFAT may falsely inflate the DXA-derived aLM measurement in individuals with a relatively high amount of adipose tissue mass (>35 % of body fat). Therefore, in this population, it is advisable to use DXA-derived aLM minus FFAT when evaluating age-related loss of skeletal muscle mass.